In general, the art has widely used copper-based alloys, such as forging brass (CDA-C37700), free-cutting brass (CDA-C36000), naval brass (CDA-C46400), high-tensile brass (CDA-C67800), aluminum bronze (CDA-C61900), and the like.
These prior art copper-based alloys are, however, not satisfactory in regard to both corrosion resistance and machinability. For example, free-cutting brass bars, forging brass bars, etc., have the disadvantage that they are susceptible to dezincification corrosion in warm water, polluted water, or sea water, because of their high zinc contents. On the other hand, while naval brass bars, aluminum bronze bars, and high-tensile brass bars are considered to be excellent in general corrosion resistance, they have poor machinability and are unsatisfactory in resistance to specific dezincification and dealuminization corrosion conditions.
Thus, in recent years, the art has proposed copper-based alloys having improved resistance to dezincification corrosion, obtained by the addition of a very small amount of arsenic to those alloys, e.g. 65/35 brass-type or 60/40 brass-type copper-based alloys, examples of which are CDA-C33530, CDA-C35330, CDA-C48600, BS2874-CZ132, and the alloys disclosed in U.S. Pat. No. 3,963,526.
However, where a very small amount of arsenic is added to such alloys, e.g. 65/35 brass-type or 60/40 brass-type copper-based alloys, to reduce dezincification corrosion, impurities in the alloys, such as iron and manganese, must be limited to very small amounts. This is because arsenic is an element with high chemical activity, and when relatively large amounts of impurities, such as iron and manganese, are contained in the alloys, the arsenic is consumed by these impurities to produce compounds thereof. Thus, the amount of arsenic available to form a solid solution, as an effective element in the substrate of the copper-based alloys, becomes insufficient, thereby making it difficult to attain the desired resistance to dezincification corrosion.
Consequently, in order to limit the contents of iron, manganese, and the like to satisfactory low levels, for example to 0.1 weight percent to 0.2 weight percent or less, recycled materials, which have been commercially recovered, must correspondingly be limited in these alloys to small amounts. This results in the necessity to use relatively large amounts of raw materials having high purity. For this reason, the material cost of such alloys is high. On the other hand, when large amounts of recycled materials are used, the amount of these impurities becomes large, and a relatively large amount of arsenic must be used to compensate for the amount of arsenic consumed by these impurities.
This approach, however, gives rise to the following disadvantages. Because arsenic is an element which can readily cause segregation into grain boundaries, the sensitivity of the resulting alloys to intergranular corrosion may be significantly increased by the deposition of arsenic compounds of, for example, iron, manganese, and the like, into the grain boundary, thereby causing severe intergranular corrosion. Additionally, in some countries, such as Japan, the use of arsenic-containing materials has been subjected to drastic restrictions, in view of safety and health considerations, and, therefore, it is preferable to avoid the addition of arsenic to these alloys.
Thus, the art has made efforts to avoid or severely limit the necessity to use arsenic in such alloys, and other elements, rather than arsenic, have been proposed for reducing dezincification corrosion of such alloys.
In regard to copper-based alloys which reduce dezincification corrosion by the addition of elements other than arsenic, Hopper's metal (U.S. Pat. No. 3,404,977) and Okano's metal (U.S. Pat. No. 4,101,317) are notable examples. These alloys improve the resistance to dezincification corrosion by the contribution of tin and nickel, both of which are added to copper-zinc alloys in a relatively large amount.
Hopper's metal, however, is a casting alloy, and it is not well adapted to hot working, e.g. hot extrusion or forging. On the other hand, Okano's metal contains 1.2 to 2.0 weight percent tin, which is a relatively high content, and, depending upon the temperature condition in a hot working step, e.g. hot extrusion, the .gamma. phase, constituted by Sn-rich Cu--Zn--Sn-type intermetallic compounds, will appear in the alloy. If such a .gamma. phase appears, the alloy will have decreased toughness and exhibit brittleness, so that cracks may readily form at the time of such hot working. Moreover, tin is prone to cause segregation, and, therefore, it is difficult to stabilize the structure of the alloy. This results in a serious drawback in that the corrosion resistance of the alloy has a tendency to vary from part to part. This difficulty can be mitigated to a certain extent by adding a large amount of nickel, by conducting the hot working within an extremely narrow temperature range, and by a heat treatment over a long period of time. However, this mitigation causes the disadvantages of, for example, significantly deteriorated operating characteristics in the production of the alloy, which becomes a problem in quality control and production yield (or cost). Furthermore, the addition of large amounts of expensive tin and nickel is economically unsound.
It would, therefore, be of significant advantage to the art to provide corrosion-resistant copper-based alloys having a stable e single phase structure, excellent corrosion resistance (especially, resistance against dezincification corrosion and intergranular corrosion), mechanical properties, and machinability, but without the necessity to use arsenic and without the drawbacks, as explained above. It would be a further advantage to provide corrosion-resistant copper-based alloys which are easy to hot work, e.g. hot forging or hot extrusion, where quality control in the production process is not a problem, with high production yields, and which alloys have stable quality at a low cost.
It would be of further advantage to the art to provide corrosion-resistant copper-based alloys which are well suited for a wide range of applications, such as valve components (e.g. body, disc, stem, etc.), machinery parts, marine equipment, electric parts, shafts, pump shafts, bushes, tube-shaped members, plate-shaped members, and the like, because the alloys have excellent resistance to corrosion caused by warm water, polluted water, sea water, or the like, and also have excellent machinability and mechanical properties.
Finally it would be an advantage to the art to provide such corrosion-resistant copper-based alloys whose working scrap, such as cutting waste, can be reutilized as a material of bronze casting and the like.